The Study of Chromium Nitride Coating by Asymmetric Bipolar Pulsed DC Reactive Magnetron Sputtering

Abstract

Polymers and carbon fibre reinforced polymer (CFRP) composites are used to replace metallic components in wide range of applications. In most of the applications, the surface of the polymers and CFRP composites that interacts require good wear resistance. Coating the surface by chromium nitride known to possess good wear resistance by Pulsed DC Reactive Magnetron Sputtering technique is one of the methods suitable for polymers having low temperature resistance. Adhesion is a challenging part while coating the polymers and polymer based composites as polymers are innately hydrophobic having low surface energy. Plasma pretreatment of polymer surfaces helps to improve the adhesion. In the present study, the following polymer based substrates were used: vinyl ester matrix CFRP composites, vinyl ester-thermosetting polymer and thermoplastic polymers such as polyamide PA), polycarbonate (PC) and polymethyl methacrylate (PMMA). The effect of ion cleaning on adhesion by pull off adhesion tester was analysed with argon ions for CFRP, and with argon and argon/nitrogen mixture for the polymers. The film stoichiometry is generally found to influence the properties of the coating. To know the effect, chromium nitride was deposited at different nitrogen partial pressure on CFRP and polymer substrates. Other than the nitrogen partial pressure, the substrate material was also found to affect the structure, mechanical properties and wear performance of the coatings. Depending on the polymer substrate material, the relationship between wear resistance and plastic deformation resistance of the coating varied. The wear performance of coatings on CFRP was not well due to the presence of carbon fibres near the surface that made the surface rough and inhomogeneous. Asymmetric pulsed DC used for the unbalanced magnetron sputtering technique for the present study is known to produce high levels of energetic ion bombardment due to the fluctuations in plasma potential, which occur during the pulse cycle. The bombardment of the substrate by these ions may have a significant impact on the growing film. The ion fluxes and energies, which impinge on the substrate during the deposition of chromium were analyzed using energy resolved mass spectrometry at different pulse frequencies. It was found that there was a remarkable increase in ion flux at higher pulse frequencies. The variation of the ion flux with pulse frequency was explained by a simple model. The behavior of the ion fluxes on the magnetron centre line and in off-axis position was compared and was found that on the centre line there was an additional mid energy ion flux, which was not present in the offset position. This change arises from the ion flow directed along the magnetic field lines from the racetrack region to the substrate or probe position because of the unbalanced magnetron configuration. Chromium nitride was coated on silicon and glass substrates at different pulse frequencies to know the effect of ion bombardment at off-axis position of the substrate on coating characteristics such as structure, morphology and mechanical properties. From distortion in structure to change in morphology, roughness, stress and mechanical properties, the ion energy bombardment was found to have a significant impact on the coating.